The present application is based on Japanese Patent Application No. 2001-103999, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an illuminator which can supply polarized light suitable for improvement in luminance of a liquid-crystal display device.
2. Description of the Related Art
As an illuminator for improving luminance of a liquid-crystal display device, there has been heretofore known an illuminator having a reflective type polarizer which is disposed on a planar light source including a side light type light pipe or a direct type light pipe so that a reflected light component and a transmitted light component of polarized light can be obtained from natural light incident on the reflective type polarizer (Unexamined Japanese Patent Publication No. Hei. 4-268505). Such an illuminator is used as a backlight in a liquid-crystal display device so that polarized light can be supplied to the liquid-crystal display device to reduce absorption loss due to a polarizer. The illuminator is generally provided with a reflection plate so that the reflected light component separated by the reflective type polarizer can be utilized effectively.
In the case where the reflected light component separated by the reflective type polarizer is utilized effectively as described above, there is theoretical expectation that luminance as twice as that in the case where the reflective type polarizer is not used can be obtained. The related-art illuminator using the reflective type polarizer, however, has a problem that luminance can be improved to be no more than about 1.4 times.
An object of the invention is to develop an illuminator using a reflective type polarizer by which polarized light can be supplied to improve luminance to be not lower than 1.5 times, especially not lower than 1.6 times as high as that in the case where the reflective type polarizer is not used.
The invention provides an illuminator which has a planar light source unit having reflectance of from 50 to 90% with respect to light in a state in which the light is emitted from a light source provided separately from the planar light source and incident on the planar light source unit, and a reflective type polarizer disposed on a light exit side of the planar light source unit so that a reflected light component and a transmitted light component of polarized light can be obtained from natural light incident on the reflective type polarizer. The invention further provides a liquid-crystal display device which has an illuminator defined above, and a liquid-crystal display panel disposed on an upper side of the reflective type polarizer in the illuminator.
According to the invention, an illuminator using a reflective type polarizer can be obtained so that luminance can be improved to be not smaller than 1.5 times, especially, not smaller than 1.6 times as large as that in the case where the reflective type polarizer is not used. The illuminator can be used as a backlight for forming a liquid-crystal display device which is supplied with polarized light, bright and excellent in display quality. The improvement in luminance is based on controlling the light reflectance of the planar light source unit to the predetermined value. In the planar light source unit which is designed to utilize light from a light source as effectively as possible to make the whole surface evenly bright as represented by a related-art backlight system, it is difficult to improve return light-utilizing efficiency by use of the reflected light component when the planar light source unit is used together with a reflective type polarizer.